1. Field of the Invention
The present invention relates to an apparatus and method for digital wireless communications using a multivalue modulation type.
2. Description of the Related Art
In a conventional digital mobile wireless communication system, a familiar example of frame configuration method to estimate a frequency offset is described in “Terrestrial Mobile Communication 16QAM Fading Distortion Compensation Method” (Sanbe, TECHNICAL REPORT OF IEICE, B-II, Vol. J-72-B-II, No. 1, pp. 7-15, January 1989). FIG. 1 shows a frame configuration according to a 16QAM system.
As shown in FIG. 1, this frame configuration has one pilot symbol inserted for every N−1 information symbols. With such a frame configuration, quasi-coherent detection is performed by estimating the reference phase, amount of frequency offset and amount of amplitude distortion using pilot symbols.
However, during quasi-coherent detection with such a frame configuration with one pilot symbol inserted for every few information symbols, symbol synchronization gets the jitters. Therefore, in quasi-coherent detection with symbols whose symbol synchronization is not completely established, the accuracy in estimating the reference phase, amount of frequency offset and amount of amplitude distortion using pilot symbols deteriorates. This results in deterioration of a bit error rate characteristic in the signal to noise ratio.
This is explained more specifically using FIG. 2A and FIG. 2B. FIG. 2A and FIG. 2B are diagrams to explain the relationship between the time and amplitude of a reception signal. In FIG. 2, reference code 1 indicates the time when pilot symbol 3 is detected with an ideal judgment time and reference code 2 indicates the time when pilot symbol 3 is detected with a time offset (jitter) generated. Reference code 4 indicates the information symbols immediately before and after pilot symbol 3.
Both a transmitter and receiver are provided with their respective clock generation functions. Because of this, the receiver has different clock generation sources, and therefore the receiver may detect waves at timing such as time 2, at which a time offset from ideal judgment time 1 has occurred. At this time, as shown in FIG. 2A and FIG. 2B, the time offset originates errors (amplitude errors) XI and XQ from the signal point. This deteriorates the error rate. Furthermore, the receiver estimates the phase, amplitude variation and frequency offset on the I-Q plane from the pilot symbol. However, when detected at time 2 when the time offset occurred, the pilot symbol signal has an error from the pilot symbol signal point, and therefore the accuracy in estimating the phase, amplitude variation and frequency offset deteriorates.